This invention relates to additives for hydraulic cements and, more particularly, additives for concretes which are to contain metal reinforcement. Such additives are added in aqueous solution to a hydraulic cement at the construction site, or to the cement just prior to being taken to the construction site.
The term "hydraulic cement" as employed herein includes those inorganic cements which when mixed with water, set and harden as a result of chemical reactions between the water and the compounds present in Portland cement. The term "Portland cement" as used herein includes those products prepared by heating a mixture of limestone and clay or shale, or other calcareous and argillaceous materials to a fused state. The fused product which is called clinker, is interground with a few percent, normally about 4 to 6 percent by weight (% by wt) of a retarder such as gypsum. The term "concrete" as used herein includes a mixture of such portland cements and inert aggregates. Typical aggregates include conventional "coarse" aggregates such as gravel, granite, limestone, quartz, etc. as well as those materials conventionally referred to as "fine" aggregates such as sand and fly ash. Conventional portland cement concretes employ major amounts, that is more than 50% and usually up to about 75% by volume of such aggregates in the set product.
Calcium chloride (CaCl.sub.2) is the most commonly used accelerator for concrete but promotes continuous oxidation of ferrous structural or reinforcing steel or aluminum conduits embedded in the concrete. CaCl.sub.2 also adversely influences the concrete ifself, resulting in spalling and weakening. Where the concrete is in contact with moist soil, the attack of sulfates in the soil is enhanced. Therefore, even where there is no metal in the concrete, or corrosion of metal in the concrete is not a serious problem, the maximum amount of CaCl.sub.2 per 94 pound (94 lb) bag of cement is no more than about 2 lb. Clearly, despite the low cost of CaCl.sub.2, the problems associated with its use prejudice its desirability in reinforced concrete construction.
Retarding mixtures are frequently not used by themselves because of the problems associated with obtaining proper control of the setting times of the concrete or mortar, as more specifically disclosed in U.S. Pat. No. 4,089,695. To overcome undesirable effects of a retarder, known accelerators such as CaCl.sub.2 and triethanolamine are used to balance out excess retardation, but the net effect of the combined ingredients of the patented additive composition is that it is a set-retarder. Unlike such a prior art composition, this invention utilizes a combination of accelerating and retarding ingredients which have the net effect of set-acceleration of a portland cement-containing composition.
One ingredient, a particular waste byproduct stream consisting essentially of a mixture of diethanolamine, triethanolamine ("DEA" and "TEA" respectively for brevity) and other byproducts, referred to herein as a waste polyethanolamine stream, belongs to the general class of alkanolamines some of which are disclosed as being accelerators in U.S. Pat. No. 2,437,842, when used by themselves in amounts in the range from 5.3% to 35.7% by wt based on the weight of portland cement. Subsequently it was discovered that neither diethanolamine nor triethanolamine were desirable accelerators because they have a weakening effect on the set concrete. At present, the use of polyalkanolamines, and particularly diethanolamine and triethanolamine are used only in admixture with other ingredients in concrete compositions because of the chemical interactions, as described in "Cement and Concrete Research," Ramachandran, V. S., 3, 41-54 (1973) and 6, 623-32 (1976). For example, polyalkanolamines are disclosed in U.S. Pat. No. 3,553,077 for their set-acceleration effect when used in combination with 20% by wt asbestos fiber and 30% by wt silica based on total solids.
The other essential ingredient of this invention, is calcium nitrate Ca(NO.sub.3).sub.2 which purports to have an ambivalent function, working either as a set-accelerator or as a set-retarder, depending upon whether it is used alone, or upon what particular set-modifying ingredients are combined with it. For example, U.S. Pat. No. 3,782,992 to Uchikawa et al. discloses that Ca(NO.sub.3).sub.2 when used alone is an effective retarder, one of the very oldest patents, U.S. Pat. No. 523,658 issued July 31, 1894, discloses that fermenting and decomposing organic matter mixed with Ca(NO.sub.3).sub.2 is an effective retarder; but Russian Pat. No. 233,505 discloses that Ca(NO.sub.3).sub.2 in combination with urea is an accelerator; and Russian Pat. No. 368,203 discloses that the combination of Ca(NO.sub.3).sub.2 with Ca(NO.sub.2).sub.2 and urea is also an accelerator. From my own tests, some of which are set forth herein in the Examples, I have found that aqueous Ca(NO.sub.3).sub.2 solution when used by itself with Atlas Type I cement gives set-acceleration with increased compressive strengths, while with Huron Type I cement, under identical conditions, the Ca(NO.sub.3).sub.2 solution provides set-acceleration and decreased compressive strength. Thus since one cannot expect to predict the behavior of cements even when Ca(NO.sub.3).sub.2 solutions are used as the sole additive, it is even more difficult to predict its behavior in combination with other additives.
Calcium nitrite Ca(NO.sub.2).sub.2 is known to be an accelerator for portland cement and is also disclosed to inhibit corrosion in reinforcing steel in U.S. Pat. No. 3,427,175 to Angstadt et al. Though this patent teaches that calcium nitrate (CaNO.sub.3).sub.2 may be present in minor amounts as a contaminant when Ca(NO.sub.2).sub.2 is produced, the quick-setting effect of Ca(NO.sub.3).sub.2 was overlooked, as was its effect on compressive strength, since, under the circumstances, these properties could not have been evident. This oversight was further reconciled because of the general expectation that nitrate salts have a retardant effect in cement, rather than a quick-set effect. For example, U.S. Pat. No. 2,673,810 discloses that potassium nitrate is an effective retarder. Uchikawa, supra, discloses that nitrates of aluminum, ammonium, calcium, magnesium, potassium and sodium are all effective retarders. Moreover, though chlorides, sulfates and nitrates are all known as ingredients of set-modifying compositions for hydraulic cements, they are all generally accepted as producing undesirable corrosion on metal reinforcements, as disclosed in U.S. Pat. No. 3,782,984 to Allemand et al.
The aqueous CaCl.sub.2 -free additive composition of my invention is particularly valuable for decreasing the setting time and increasing the nearterm compressive strength, as well as the extended compressive strength of portland cement compositions and particularly portland cement concrete. Portland cement is the most commonly used hydraulic cement such as is designated by the specification set forth in ASTM C150-74. Many prior art references teaching additive compositions for cement fail to recognize the essentiality of excluding calcium chloride from a concrete composition suitable for use with metal reinforcing.
Thus, U.S. Pat. No. 4,116,706 to Previte teaches that a set-accelerating and compressive-strength enhancing composition for hydraulic cement may contain a major amount of a chloride, formate or nitrite salt set-accelerating agent, from about 1 to about 6% by wt of an amine, and from about 0.15 to about 3% by wt of a synthetic surface active agent (surfactant) having certain characteristics. Though the surfactants disclosed appear to contribute to the compressive strength of the concrete, there is no teaching as to their effect on retarding the oxidation of metal in the concrete if chloride salts are used.
Since the particular polyethanolamines DEA and TEA are known to provide desirable acceleration, as does monoethanolamine, the on-going challenge is to find a particular combination of ingredients in an additive composition which provides desirable set-acceleration and at the same time enhances compressive strength compared with concrete without the additive composition. Moreover, whether or not a specific alkanolamine is compatible with a co-accelerator is not predictable, and an overly broad disclosure with respect to the general usefulness of alkanolamines as co-accelerators is not only misleading but also unavoidably suggests combinations which are either ineffective set-accelerators, or which weaken the set concrete, or both.
Thus, to date, I am unaware of any additive composition for portland cements which utilize the combination of calcium nitrate with diethanolamine or triethanolamine, or a mixture of polyethanolamines, preferably admixed with an alkali metal or alkaline earth metal salt of a polyhydroxy aliphatic compound, or of lignosulfonic acid. From the prior art, I know of no reason to expect that such a combination would have such desirable effects on setting time and compressive strength. Moreover, particularly as alkanolamines are generally soluble in water or in aqueous salt solutions, but a waste polyalkanolamine stream comprising a major amount of triethanolamine is difficultly soluble in a concentrated aqueous solution of calcium nitrate, such a combination of a waste stream with Ca(NO.sub.3).sub.2 appears to be ill-suited for a role as a practical and commercially attractive aqueous additive composition, stable enough for use in construction particularly during winter months.